Cyanidation method and apparatus



Dec. 12, 1944.

1 D. MILLS El'AL CYANIDATION METHOD AND APPARATUS 4 Filed July so, 19402 Sheets-Sheet l F'I E l INVENTORS [cu/s D. Mil/s Thomas 5. Crowe BYfaymondifiy/el ATTORNEY 1944. L. D. MILLS ETAL CYANIDATION METHOD ANDAPPARATUS 2 Sheets-Sheet 2 Filed July 30, 1940 BY .G

Patented Dec. 12, 1944 CYANIDATION METHOD; AND APPARATUS Loui'sD. Millsand .ThomasB. Crowe, Palo Alto, andRaymond E. Byler, StanfordUniversity,

Calif., assignors to, The Merrill Company, San Francisco; Calif., acorporation of California i I Application July so, 1940, Serial. No.348,378

4 3 Claims; (01. 210-153) This invention rel-atesgenerally toi methodsand. apparatus for clarifying liquids, as for example pregnant cyanidesolutions; used in-the treatment of gold silver ores. More specifically,

it relates to methods andapparatus of the type Wherein. asubmergedvacuum leaf filter is utilized for the purpose ofclarifying thesolution immediately prior to deaeration and precipitation. 7

In cyanidation systems utilizing continuous clarification of the cyanidesolution followed by deaeration and precipitation of the same, it hasbeen proposed to condition. or vp-recoat the filter cloth of theindividualleaves of the clarifying filter following removalv offiltercakefrom the same, and before the leaf hasbeen reconnected withthe filter efiluent manifold, in order to insure proper filtration ofthe solution. This conditioning. operation; has been carried out bycontinuously circulating thecyanide solution. through a filter leafwhile' it'is detached from the effluent manifold, until the filtrateattains proper clarity, after which the leaf is connected for normaloperation. While one may depend upon ore solids of the solution forconditioning the, surface of the filter cloth, it is preferabletomake-use of a. filter aid-such as diatomaceous silica to form a precoat.

The-economical use of such-methods and the advantages to begainedzbyapplying a. precoat are dependent to a substantial. degree uponthe uniformity or lack of uniformity of-the. applied coating. In'general, lack of uniformity of the precoat makes for increasedconsumption of the filter aid'and requiresan undue extension of theprecoat periodfor the purposeof insuring coating of all thesurfacesofthe filter cloth.

Where a certain-minimum coating is not .pro-

vided for certain areas or patches: of the-filter cloth, theslimeaccumulated during a normal filtering operation. will not wash offreadily; and the precoat may not .function properly to insure asparkling filtrate. Also, slime penetrates into the pores of the filtercloth and, by choking the' pores; reduces the useful lifeof the cloth.In

addition, where -a filter. aid like-diatomaceous" silicia is beingemployed, contamination of the precoat with slime solids interferes withits effectiveness, as by increasing the resistance to flow of solutionand. by interfering with washing.

off the filter-cake.

A-nobject of the present invention is to proe vide amethod; andvapparatus which will produce aprecoat upon the. filter leaves of optimum.-uniformity, thereby avoiding, a coating of undue .55

thickness upon certain. areastof the filter membrane,- or areas,insufiicientl-ycoated.- with the filter aid. In attaining this=object,theinvention is characterized byapplication of-suction-to the twolaterally spaced ends of iaz fi1ter1eaf5during the conditioning .or.precoating operation. therebyi'nsuring substantially; same flow rate.

throughall of the. various portionsnof the filter cloth while. theprecoati's: being applied.

Another object of the'invention .is to introduce theclarifiedisolution": into thewbottomz of the precoat compartment. in:such away: as to promote uniform suspension of' the precoating material.Another object of the invention is to. insure'a Drecoat. of a filter'aidlike-diatomaceous earth without substantial" contamination with.slime. solids'such as are ordinarily present in' unclarified cyanidesolution. Ingeneral, this is:ac complished by means whereby during avprexcoat solution is withdrawn fromthe leaf and is re- When the leaf istransposed into operating .position inthe filter tank, effluent orclarified solution is also withdrawn from the leaf and. is returned tothe bottoi'n of the precoat compartment, with" a small portion beingreturnedito the top of the precoatrompartment for visual inspection,to'insure claritybeforeconnecting the leaf to deliver efiluentinto thenormalsuction manifold of the-filter! Further objects of'th'e inventionwill appear "from the'following description in which the preferred'emb'odiment'has been set; forth in detail in conjunction with theaccompanyingdrawings. Referring ,to the drawings:

Fig. 1 is a diagrammaticview illustrating a vacuum leaf clarifying:filter. connected with a solution deaerating receiver; g

Fig. 2- is a diagrammatic elevation illustrating apparatus incorporatingthe present invention, including a separate precoat container or corn-.partment into which a. filter leaf is, transferred during a precoatingoperation; I

Fig. 3 is a diagrammatic view showing the same apparatus as Figs. 1 and2,, but looking toward one side of the precoat compartment, with aportion .of the tank out .away'to show details of the leafconstruction;v

Fig. i is. a detail illustrating a modification in which the precoatcompartment is integral. wit the clarifying tank;

turned to the bottom of the same compartment. 1

solution having dissolved precious metal values 1 is supplied to thetank of the clarifying filter H1, and after being clarified, theeffluent flows through the evacuating receiver H where dissolved air isremoved. This solution then flows from the deaerating receiver tosuitable precipitating apparatus, which may include means forintroducing a precipitant like zinc dustinto the solution, and asuitable precipitating filter.

The clarifying filter H] is of the vacuum leaf type and consists of asolution tank M in which a plurality of leaves I6 are disposed. Eachfilter leaf is provided with a pair of riser pipes l1 and I8, whichcommunicate with the two laterally spaced ends of the filter leaf, andwhich have lateral extensions Ila and Ba. Extension |'|a is providedwith a coupling IQ for making detachable connection with the 'efliuentmanifold 2|,

and also with a stop cock 22. Between the couvpling l9 and manifold 2|,there is the usual valve 23. The extension I 81: on riser pipe I8 isprovided with a valve 24 and coupling means 25.

Solution is introduced into the main compart- 7 ment of tank M by thesolution line 21, which can be controlled by float valve 23. Flow line29 serves to connect the effluent manifold 21 with the upper portion ofthe deaerating receiver II. This line is shown being controlled by thevalve 3|], which in turn is controlled by a float 3| in the deaeratingreceiver. Clarified and deaerated solution is removed from receiver H bypipe 32, which can connect to the suction side of suitable pumpingapparatus. The receiver H is evacuated by pump 33. When a filter leaf isin operation, valve 24 is closed, and

valves.22 and 23 are open, so that the efiluent passes directly throughline 29 to the deaerating receiver.

For carrying out the precoating or conditioning operation onan'individual leaf, it is desirable to make use of a separate precoatcontainer or compartment 34, which is shown disposed at the same levelas the filter tank. Suction means is utilized in conjunction with thecontainer 34, and includes a pump 35 having its discharge side connectedto the pipes 36 which deliver solution intothe lower corners of theprecoat container, and also pipes 31 which deliver solution through thebottom of the container. These pipes are shown being controlled byvalves 36a and 3111., An auxiliary pipe 38, controlled by cock 38a andconnected to one of the pipes 36,. discharges into the upper portion ofthe precoating compartment above the solution level to enable visualinspection of the solution as it is being discharged. A check; valve 40-is provided in the discharge from pump 35 to prevent ,backflow throughthe pump and into the separator 4|. or chamber, in which solutionflowing into the same fromv a filter leaf separates out from any airwhich may be withdrawn from the leaf at the same time, with the solutionbeing returned to the pump 35, as will be presently explained.

Above and opposite the entrance of pipes 36 into the bottom of thecompartment 34, are the This separator is a small tank deflectors rbaflies 31b which serve to deflect solution flow in a lateral direction,and thus prevent direct impingement of liquid discharged from pipes 31upon the lower edge portion of the leaf.

In conjunction with the precoat compartment, attachments.v are provided.for connecting both couplings l9 and to the suction side of the pump35, and in conjunction with these attachments, means is provided forinsuring continued application of suction to the leaf while it is onlpartly immersed or entirely out of the solution. This means issubstantially as disclosed in Mills and Crowe patent No. 2,261,399.Thus, pipe 39 connectsthe intake side of .pump with the lower end ofthe-separator 4|. Pipe 42 connects with the upper part of separator 4|,and leads to suitable evacuating means such as an exhaust pump 43.municate with separator 4|. Pipe 44 is provided with a flexible hosesection 46 for attachment to the coupling 25 of -a filter leaf. Pipe isprovided with a vertical pipe section 41 for attachment with coupling I5of the leaf.

The interior of separator 4| is provided with a float 48 carried by thevertically movable valve rod 49. The upper end of rod 49 extends througha vent 5| in "separator 4|, and is provided with a valve closure 52whereby when solution Within the separator and received from pump 35rises above a given level the float 48 opens closure 52 to permit air tobe drawn'in from the atmosphere, thus reducing the vacuum within theseparator. During intervals when the leaf is only partly immersed, airis drawn into the leaf together with solution and is discharged into theseparator 4| from which it is withdrawn through suction pipe 42. 1

Each-of the filter leaves l6 includes generally a frame enclosed by thefilter cloth I611. The lower edge of this frame is formed by thehorizontal pipe 54, which connects with the lower ends of the riserpipes l1 and I8. The top hori- 1 zontal member or header55 connects theupper portions of the riser pipes l1. and |8, and the spacers or splines56'extend between the lower pipe 54 and the header 55. Pipe 54 isprovided with effiuent openings 51, and riser pipe I8 is provided withan opening 58 which serves as an air vent, and which is locatedimmediately below' the header 55. I

A filter leaf is attached to pipe 4'! only during the precoatingoperation and while the leaf is within the precoating compartment. onthe other hand, a leaf is attached to the flexible hose 46 during the.precoating operation, and also durwhereby the leaf is connected to thefixed pipe 41 during the p-recoating operation,'the upper edge of theprecoat compartment is shown providedwith a resilient compressiblemember 6|, which may be a short, length of rubber hose of suitabledimensions. At the other end of the precoat compartment the leaf may besupported in any desired manner, as'by means ofhaving the pipe extension|8a rest within a shallow notch or groove formed in the upper edge ofthe precoat compartment.

Suction pipes 44 and 45 also com- In-Fig'. z' theprecoat-compartment'fliis shown w pump- 35 is-in the main filtentank; and thepump is in operation; solution discharged by 1 "the filter leaf duringthe precoat operation, is

' inpartition 63' corresponds t'o' the flovf'pipe 62 of "Fig 22near-'tlie pump-35, orin'the'inlet 'passage of jthis"3n offthe filterleafr but withitheseidefiectorsuthere is-no direct impingement uponwthelower .ed'ge connected tothemain filter tank= l4 by the pipe connection62. 'When'the filter'leaf connected pump 35 into the -p'reeoatcompartment 3'41 flows through pipe 62 back to-tliemainfilter tank.

It will'be evident that because-of theconnectionswitl'i themain tank;the solution ii1'';tlie1precoat tank 34* andwhiclriscirculated throughthe same as'themyanid'e solution being in the main tank.

i the precoat compartment is an integralpart of the-main-filter tank. Inthis instance a'partition' 63 is= disposed near one end. of the mainfilter tank to form-aseparateprecoat compartment-M; corresponding tozthecompartment 'ilfi' of Figs. 1 to' 3 inclusive. An overflow-opening 56 InI order" to make certain that the pump is properly primed at all times,a pipe 61- can be pipe connecting tothe pump housing adjacent the inletopening of 1 the same; and tlie other: end

' connecting'to the upper part of the" separator 41. Any ai'r whichmaylbe'trapped in pipe 39 pump; isfreeto vent through" .pipe '61-.

Use'of the apparatus described: above and the carrying out of thepresent method' can be ex- -plained as-follows: When a considerablelayer f of slime has accumulated upon a filter leaf (with '35,: "theparts as shown in Fig. l),xma.kingit desira- I ble to r'econdition aleaf for further use, the corresponding Valve 23 is closed, andcouplingis disconnected. If the accumulated filter cake -is of such a characterand the filter so constructed 40' that the leaf can beraised withoutdisturbing the cake; ofwithout-injuring the filter leaf, it isimmediately removed from' the main filter tank as is now conventionalpractice. However, in instances where the filter membrane tends to bagnear the lower end of the leaf while being removed, thus disturbing andcausing possible sloughing off of the slime cake, itis desirable tofirst connect hose 46 with the coupling 25, and start pump 35 inoperation (with valve 24 open 50 and valve 22 closed), whereby suctionis placed upon the interior of the leaf while it is being raised fromthe filter tank. Such application of suction scavenges the interior ofthe leaf of solution and consequently prevents sagging of the lowerportion of the leaf with resulting sloughing off of filter cake orpossible injury to the filter cloth. The accumulated filter cake is nowwashed from the surface of the leaf as by methods now in practice onfilters of this char- 6Q acter. A proper quantity of filter aid such asdiatomaceous silica is added to the solution in the precoat container34, and the clean filter leaf is now transferred to this container, asillustrated in Figs. 2 and 3. The operator now connects the pipe 41 withthe coupling I9, and with valves 36a and 31a open and valve 380. closed,the pump 3'5 is started in operation.

Discharge of circulated solution through pipes 35 and 3! into the lowercorners of the precoat container causes sufficient agitation within thecontainer to maintain the filter aid in proper suspension. Without thebaffles or deflectors 3'"), solution discharging from the pipes 31 wouldtend to wash away filter aid from adjacent areas tained above pipe 54;

portion 0f the leaf:

' As filtrate K is withdrawn through- 4 both of -the riser-'pipes l WandI8,- a coating of thetfilter aid rapidly accumulates: upon: all of lthe: surfaces of' thefiltercloth; and the thickness. ofathe coat ing isrelatively uniform for all areas; Unif'ormity" of the-=coating is--enhancedby application of equal i suction to: both the riser pipes H vand is; since: it has 1 beem found: that: application' of=suction-to:only .one=riser pipeztendssto cause application: of [atthicken coating of." the filter: aid in regionsnear: that J end of the:filter leaf; Reintroduction. of: eflluentr throughthe pipes 31 assistsin keeping theifilter'aidz'uni'form- 1y suspended; and also contributestoiproduction of; uniform .coating-of -the filter aidi. Without the"-names 31b portions of: the-filter directly 'above ipes 3:1 wouldm'notreceive aproper 'ap- I plioation' of. filter aid; due:toyimpingementrpofsolution: jetting: from these; pipes: Bafflesr 31bpreventrsuch:impingement; without' however. :de

' tractingrfrom;maintenanceaof filtenaid in proper provided as-shown inl 'ig. 2, withpneend of this 5 suspension; Withrespectutmtheioperationofipump35, .ittis desirablethat this pumpranditsconnected piping: and :hose have such :a; capacity that. no. col-.umnof; solution ispermittedato accumulate-above thev perforated: pipes54; This servesv to avoid variations in. pressure-- differential across;the filter. membrane for different heights such. as would existifi a:columnof filtrate were'main- Within a comparatively short period oftime,

' the applicationsof filter aid is-completed, andthis ismade evident; byendearing: of: the solution in. the precoat: compartment. The; operatornow transfers; the conditioned filterleaf back, into the-main filter;tank. It is: desirable during; this operation to maintain suction uponthe-leaf,- and thereforethe'operator; closes valve-:22abeforepipe Mb iszuncoupled [but maintains the connection with hose section 48 while theleaf is being transferred to and lowered into the main filter tank.-'After'being lowered into normal operating positions, and assuming thatpump 35 re mains in operation, the operator inspects effiuent dischargedfrom pipe 38 by opening valve 380;, and when this efiluen't is of properclarity, coupling 19 isconnected to the efliuent manifold 2|; valves 22and '23 are opened; and then valve 24 is closed; and coupling 25 isdisconnected from the hose section 46. The filter leaf has now beenreturned to normal operation. It will be evident that effluent from pipe'38 under such conditions is a part ofv the discharge from pump 35 andtherefore is a part of the effluent being withdrawn from the filter.leaf.

It will be apparent that since the method and apparatus described abovemakes possible the uniform application of filter aid, the thickness ofthe coating can be maintained at a minimum, consistent with the resultsdesired. Therefore,

.all wasteful consumption of filter aid is avoided.

In addition, the time required for application of the precoat is reducedto a minimum, and the danger of having areas or patches of the leafimproperly coated with the filter aid is entirely obviated. A y

A further feature of the invention is that there are substantially noslime solids in the solution during a preooat operation; and thereforethe coating of filter aid is not appreciably contaminated with slimesolids such as are ordinarily Q4,- I I found in unclarified cyanidesolution. Any solu- 1 tion flow through the pipe connection 62 of Fig.2, or the openings 66 of Figs. 4 and 5, is normally 'from the precoatcompartment into the main f -filter tank, and only at rare intervalsis'there a slight back fiow of solution from the main tank I back to theprecoat compartment to equalize solution levels.

' Check valve 40 prevents any back flow of filter aid and solution fromthe precoat compartment through pump 35 and into the separator 4|,

of filter aid remain in the precoat compartment.

Should filter aid enter separator 4|, it would interfere with subsequentinspection of the efiluent to determine proper clarity.

We claim:

tion preparatory to deaerating and precipitating the same, a filtertank, a plurality of vacuum filter leaves suspended within said tank,each leaf having two separate pipes connecting to the laterally spacedends of the leaf, each pipe having avalve and a coupling means formaking connection with external suction means, a separate precoatcompartment into which a leaf can be in- ..troduced, said compartmentbeing adapted to should pump 35 stop while appreciable amounts 1. Amethod forthe conditioning of the indi- I vidual filter leaves of avacuum filter employed for the clarification of cyanide solution, whereduring a normalfiltering period the leaves are connected to a commoneflluent line to'deliver filtrate to an evacuated deaerating receiver,the

steps of disconnecting an individual filter leaf 1 from the efiluentline and lifting the same from the filter tank, removing accumulatedslime from 1 the leaf, introducing'the leaf into a separate precoatcontainer in which there is cyanide solution suction to both of the twolaterally spaced ends the filter aid to coat the filter membrane, dis- 1continuing application of suction to one end of the leaf and thentransferring the leaf back to I together with a filter aid insuspension, applying so of the leaf for a period of time sufiicient tocause contain solution together with a filter aid, means for applyingsuction to one of the pipes of a filter leaf when the leaf is in normalposition within the filter tank and for applying suction to both saidpipes when said leaf isremoved from the filter tank and within theprecoat compartment.

3. In apparatus of the character described for conditioning vacuumfilter leaves used in the coat compartment or container, the compartmentbeing adapted to contain solution together with a filter aid, pumpingmeans, a flexible hose for connecting the intake of the pumping means toone of the outlets of the filter leaf, whereby sucvtion can be appliedtoa filter leafv while being transferred between the filter tank and thepreis continued to the other end of the leaf until,

the effluent attains proper clarity, and'then reestablishingcommunication between the one end of the leaf and the common effluentline and interrupting application of suction to said other end of thefilter leaf.

2. In apparatus for conditioning vacuum filter I leaves used in theclarification of cyanide solucoat compartment, and an additional pipeconthe main-filter tank while application of suction-- :3

nection from the intake of the pumping means to the other outlet of thefilter leaf whereby suction can be applied separately to the twolaterally spaced ends of the filter leaf while within the precoatcompartment.

LOUIS D. MILLS. THOMAS E. CROWE. RAYMOND E. BYLER'.

